The present invention relates to a lithium secondary battery and a method of manufacturing the same. More specifically, the invention relates to a metal jacket of a lithium secondary battery made of a magnesium-based alloy containing lithium (Mgxe2x80x94Li alloy).
With the recent prevalence of portable apparatuses, demands for secondary batteries have been increased. In particular, a lithium secondary battery containing an organic electrolyte, which enables a reduction in the size and weight of such a portable apparatus, has obtained a rapidly increasing share in the market. Though the majority of conventional lithium secondary batteries have cylindrical or coin-like shapes, the number of secondary batteries having rectangular shapes have begun increasing recently. In addition, sheet-like thin batteries have made their debut.
It is very important to increase the energy density of a battery. The energy density of a battery can be expressed by volume energy density (Wh/liter), which indicates the size of a battery, and weight energy density (Wh/kg), which indicates the weight of a battery. From the viewpoint of a reduction in size and weight, a battery is required to have a higher volume energy density and weight energy density, because a keen competition exists in the market of such batteries.
The energy density of a battery is determined mainly by active materials of the positive and negative electrodes as the power generating elements. Other important determinants include the electrolyte and the separator. Improvements in these determinants are being intensively made in pursuit of a battery having a higher energy density.
A metal jacket for accommodating such power generating elements is also reviewed as an important factor of a reduction in size and weight of a battery and is being improved actively. If the metal jacket has a thinner wall, larger quantities of the active materials can be accommodated within the metal jacket in a conventional shape. This leads to an improvement in the volume energy density of the battery. Alternatively, if the weight of the metal jacket can be reduced, the weight of the battery in a conventional shape can be reduced. This leads to an improvement in the weight energy density of the battery.
One known example of a battery with a light metal jacket is a lithium ion battery in a rectangular shape that employs a metal jacket made of a light aluminum-based alloy sheet (specific gravity: about 2.8 g/cc) instead of a conventional steel sheet (specific gravity: about 7.9 g/cc). In the technical field of batteries for use in cellular phones, there is known a case where the weight energy density of a battery has been increased by about 10% by employing the metal jacket made of an aluminum-based alloy (refer to JAPANESE PATENT LAID-OPEN GAZETTE No. HEI 8-329908).
Many methods of manufacturing such metal jackets made of aluminum or aluminum-based alloy have an impact process or a drawing process.
Attention has recently been focused on magnesium-based alloys, which are lighter than aluminum or aluminum-based alloys. The specific gravity of magnesium is 1.74 g/cc, whereas that of aluminum is 2.7 g/cc. Examples of well-known magnesium-based alloys include alloys comprising magnesium admixed with Al, Zn or the like. Some cases are known where a magnesium-based alloy is used for the metal jacket of a battery (refer to JAPANESE PATENT LAID-OPEN GAZETTE Nos. HEI 11-25933 and HEI 11-86805).
Further, attention has recently been paid to magnesium-based alloys containing lithium which have superplasticity (refer to JAPANESE PATENT LAID-OPEN GAZETTE No. HEI 6-65668). Magnesium-based alloys containing lithium are characterized in that they have smaller specific gravity (about 1.3 to 1.4 g/cc) than pure Mg and are superior in mechanical workability to conventional magnesium-based alloys containing Al.
There is, however, not known any case where a magnesium-based alloy containing lithium is applied to a metal jacket for a battery.
Meanwhile, thixomolding process is receiving attention as a novel technique for processing magnesium-based alloys in the art of structural material for use in various electric appliances. Thixomolding process is a modification of the diecasting process, which has been the mainstream of the conventional technology, and is similar to the injection molding process for plastics. Specifically, this process injects a raw material alloy in a semi-molten state into a mold, solidifies the raw material alloy, and then removes the molded product from the mold. The resultant crystal of the molded product is not of a dendritic structure, which results from the diecasting process, but of a granular structure, which results from the solidification process under stress. The alloy having a granular structure exhibits such features as improved mechanical properties and stabilized quality even when made thin.
There is, however, not known any case where a magnesium-based alloy containing lithium obtained by thixomolding process is applied to a metal jacket for a battery.
Though there are some cases where magnesium-based alloys are used for metal jackets (JAPANESE PATENT LAID-OPEN GAZETTE Nos. HEI 11-25933 and HEI 11-86805) as described above, the conventional magnesium-based alloys have poor workability and hence have been difficult to be applied to metal jackets for batteries practically. Further, since magnesium-based alloys are corroded when brought into contact with a power generating element such as an electrolyte, the use of a magnesium-based alloy has not been practical in view of realizing a satisfactory charge/discharge cycle.
The present invention has been made to provide a lithium secondary battery having a higher capacity and a lighter weight than the prior art battery. To this end, the present invention uses, as the raw material of a metal jacket, a specified magnesium-based alloy containing lithium (Mgxe2x80x94Li alloy) that can be subjected to a processing of bending, deep drawing or the like in the cold work, the process having been considered difficult for conventional magnesium-based alloys. In one embodiment of the present invention, a light and high-strength lithium secondary battery of high quality is manufactured by the use of a sheet of a magnesium-based alloy containing lithium obtained by the thixomolding process.
In the present invention, the metal jacket is prevented from corrosion due to contact with the electrolyte or the like by forming a metal layer or an insulating layer integrally with the metal jacket on the inner wall thereof. As a result, it becomes possible to realize a stabilized charge/discharge cycle, which has been considered impossible to realize for a battery having a metal jacket made of a magnesium-based alloy.
Among light batteries, in the case of a battery using a metal jacket made of an aluminum or aluminum-based alloy, the negative electrode of the battery cannot be connected to the metal jacket. This is because the connection between the metal jacket and the negative electrode would facilitate the production of an intermetallic compound such as AlLi that makes the metal jacket brittle. The majority of conventional batteries, however, have a structure in which the metal jacket is connected to the negative electrode. From the viewpoint of obtaining a general-purpose battery, it is desired that the metal jacket should be electrically connected to the negative electrode.
The metal jacket of the battery in accordance with the present invention, in contrast, is free from an inconvenience such as embrittlement even when electrically connected to the negative electrode by virtue of the metal layer or the insulting layer formed integrally with the metal jacket. Thus, the battery of the present invention is superior in terms of versatility also.
The present invention is directed to a lithium secondary battery comprising an electrode assembly and a non-aqueous electrolyte, both accommodated in a metal jacket, wherein the metal jacket is made of a magnesium-based alloy containing lithium in an amount of 7 to 20% by weight; and a metal layer is formed integrally with the metal jacket on the inner wall thereof for preventing corrosion of the metal jacket. The electrode assembly comprises a positive electrode, a negative electrode and a separator in general.
In this battery, the magnesium-based alloy containing lithium preferably contains lithium in an amount of 7 to 15% by weight, and at least one element selected from the group consisting of Al, Zn, Mn, Zr, Ca, Si, and rare earth elements in a total amount of 0.3 to 5% by weight.
Alternatively, the magnesium-based alloy containing lithium may be a binary alloy containing lithium in an amount of 12 to 16% by weight.
The metal layer for preventing corrosion of the metal jacket preferably comprises Ni or Cu.
Further, the metal layer is preferably formed by cladding, plating or vapor-deposition.
The present invention is also directed to a lithium secondary battery comprising an electrode assembly and a non-aqueous electrolyte, both accommodated in a metal jacket, wherein the metal jacket is made of a magnesium-based alloy containing lithium in an amount of 7 to 15% by weight, and at least one element selected from the group consisting of Al, Zn, and Mn in a total amount of 0.3 to 5% by weight; an Ni layer having a thickness of 2 to 20 xcexcm is formed integrally with the metal jacket on the inner wall thereof by cladding; and the metal jacket is electrically connected to a negative electrode in the electrode assembly.
In this construction, the magnesium-based alloy containing lithium is preferably produced by thixomolding.
Preferably, the metal jacket is in a shape of a bottomed can with an open top, having a bottom/side wall thickness ratio (bottom wall thickness/side wall thickness) of 1.1 to 2.0, and the magnesium-based alloy containing lithium is produced by thixomolding.
The present invention is also directed to a method of manufacturing a lithium secondary battery, comprising the steps of: (1) preparing a sheet of a magnesium-based alloy containing lithium in an amount of 7 to 15% by weight, and at least one element selected from the group consisting of Al, Zn, and Mn in a total amount of 0.3 to 5% by weight by thixomolding; (2) forming a Ni layer integrally with the sheet on at least one face thereof by cladding; (3) forming a metal jacket in a shape of a bottomed can with an open top with the Ni layer formed on the inner wall thereof from the sheet by a mechanical processing selected from drawing, combined drawing and ironing, and impact; and (4) placing an electrode assembly and a non-aqueous electrolyte into the metal jacket.
The present invention is yet also directed to a lithium secondary battery comprising an electrode assembly and a non-aqueous electrolyte, both accommodated in a metal jacket, wherein the metal jacket is made of a magnesium-based alloy containing lithium in an amount of 7 to 20% by weight; and an insulating layer is formed integrally with the metal jacket on the inner wall thereof.
In this construction, the magnesium-based alloy containing lithium preferably contains lithium in an amount of 7 to 15% by weight, and at least one element selected from the group consisting of Al, Zn, Mn, Zr, Ca, Si, and rare earth elements in a total amount of 0.3 to 5% by weight.
Alternatively, the magnesium-based alloy containing lithium may be a binary alloy containing lithium in an amount of 12 to 16% by weight.
The insulating layer preferably comprises a metal oxide or a resin.
The present invention is still also directed to a lithium secondary battery comprising an electrode assembly and a non-aqueous electrolyte, both accommodated in a metal jacket, wherein the metal jacket is made of a magnesium-based alloy containing lithium in an amount of 7 to 15% by weight, and at least one element selected from the group consisting of Al, Zn, and Mn in a total amount of 0.3 to 5% by weight; and a resin layer having a thickness of 5 xcexcm or more is formed integrally with the metal jacket on the inner wall thereof.
In this construction, the magnesium-based alloy containing lithium is preferably produced by thixomolding.
Preferably, the metal jacket is in a shape of a bottomed can with an open top, having a bottom/side wall thickness ratio (bottom wall thickness/side wall thickness) of 1.1 to 2.0, and the magnesium-based alloy is produced by thixomolding.
The present invention is still yet directed to a method of manufacturing a lithium secondary battery, comprising the steps of: (1) preparing a sheet of a magnesium-based alloy containing lithium in an amount of 7 to 15% by weight, and at least one element selected from the group consisting of Al, Zn, and Mn in a total amount of 0.3 to 5% by weight by thixomolding; (2) forming a resin layer integrally with the sheet on at least one face thereof; (3) forming a metal jacket in a shape of a bottomed can with an open top with the resin layer formed on the inner wall thereof from the sheet by a mechanical processing selected from drawing, combined drawing and ironing, and impact; and (4) placing an electrode assembly and a non-aqueous electrolyte into the metal jacket.
The present invention is further directed to a method of manufacturing a lithium secondary battery, comprising the steps of: (1) preparing a sheet of a magnesium-based alloy containing lithium in an amount of 7 to 15% by weight, and at least one element selected from the group consisting of Al, Zn, and Mn in a total amount of 0.3 to 5% by weight by thixomolding; (2) forming a metal jacket in a shape of a bottomed can with an open top from the sheet by a mechanical processing selected from drawing, combined drawing and ironing, and impact; (3) forming a resin layer integrally with the metal jacket on the inner wall thereof; and (4) placing an electrode assembly and a non-aqueous electrolyte into the metal jacket.
While the novel feature of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.